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United States Patent |
5,340,776
|
Paschke
,   et al.
|
August 23, 1994
|
Preparation of very fine glass powder of high purity
Abstract
For the preparation of high purity glass powder having a mean particle size
of .ltoreq.10 .mu.m, glass powder having a larger particle size up to 300
.mu.m is ground to the desired particle size in a stirred mill with glass
grinding elements in the presence of a grinding liquid comprising water or
preferably a mixture of at least 50% by weight of water and at least one
water-soluble, oxygen-containing organic compound having 1 to 5 carbon
atoms in the molecule, e.g., tert.-butyl alcohol; the ground slurry is
then frozen, and the solvent is subsequently removed from the frozen
slurry by freeze-drying. A resultant glass powder with a mean particle
size d.sub.50 of 0.5 to 2 .mu.m is particularly suitable as a filler for
synthetic resins in the dental sector.
Inventors:
|
Paschke; Hartmut (Ergolding, DE);
Daimer; Johann (Oberahrein, DE);
Haring; Richard (Landshut, DE)
|
Assignee:
|
Schott Glaswerke (Mainz, DE)
|
Appl. No.:
|
881929 |
Filed:
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May 12, 1992 |
Foreign Application Priority Data
Current U.S. Class: |
501/11 |
Intern'l Class: |
C03C 003/00 |
Field of Search: |
501/11
241/15,17,26,21
65/61
264/28
|
References Cited
U.S. Patent Documents
3126293 | Mar., 1964 | McSheehy et al. | 241/15.
|
3358938 | Dec., 1967 | Browal | 241/21.
|
3423032 | Jan., 1969 | Eckert | 241/15.
|
3881661 | May., 1975 | Powers et al. | 241/15.
|
4056230 | Nov., 1977 | Decobart | 241/15.
|
4202815 | May., 1980 | Wegmann | 260/42.
|
5047182 | Sep., 1991 | Sundback et al. | 264/28.
|
Other References
James S. Reed, Introduction to the Principles of Ceramic Processing, Wiley
& Sons 1988 (no month) pp. 53, 126-127, 255-258, 268-271, 274, 423.
|
Primary Examiner: Bell; Mark L.
Assistant Examiner: Wright; A.
Attorney, Agent or Firm: Millen, White, Zelano & Branigan
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of application Ser. No.
07/819,237, filed Jan. 13, 1992.
Claims
What is claimed is:
1. A process for the preparation of very fine glass powder of high purity
having a desired mean particle size d.sub.50 of .ltoreq.10 .mu.m by
wet-grinding in the presence of grinding elements, comprising grinding a
glass powder having a higher mean particle size than 10 .mu.m and a
maximum particle size of .ltoreq.300 .mu.m to said desired mean particle
size d.sub.50 of .ltoreq.10 .mu.m in a stirred mill with glass grinding
elements, in the presence of a grinding liquid comprising water or a
mixture of at least 50% by weight of water and at least one water-soluble,
oxygen-containing organic compound having 1 to 5 carbon atoms in the
molecule; freezing the ground slurry; and freeze-drying the frozen ground
slurry to remove the grinding liquid.
2. A process according to claim 1, wherein the grinding elements have the
same composition as the glass to be ground.
3. A process according to claim 1, wherein the grinding elements have a
size of from 0.3 to 10 mm.
4. A process according to claim 1, wherein the grinding elements have a
size of from 0.5 to 2 mm.
5. A process according to claim 1, wherein the grinding elements are
cylindrical.
6. A process according to claim 1, wherein the grinding liquid has a pH of
from 1 to 12.
7. A process according to claim 6, wherein the grinding liquid has a pH of
from 2 to 6.
8. A process according to claim 6, wherein the grinding liquid has a pH of
from 8 to 12.
9. A process according to claim 6, comprising adjusting the pH with a
compound selected from the group consisting of hydrochloric acid, nitric
acid, ammonia and ethylamine.
10. A process according to claim 1, wherein the grinding liquid is a
mixture of 80-99% by weight water and 1-20% by weight of a compound
selected from the group consisting of methanol, ethanol, n-propanol or
i-propanol, acetone and tert.-butyl alcohol.
11. A process according to claim 1, wherein the mill comprises or is coated
with (i) an abrasion-resistance plastic, or (ii) with a glass having
substantially the same properties as the glass to be ground, on at least
parts of the mill contacting the powder.
12. A process according to claim 11, wherein said abrasion-resistant
plastic is reinforced with a filler selected from the group consisting of
glass powder and glass fibers, the reinforcing glass having substantially
the same properties as the glass to be ground.
13. A process according to claim 1, further comprising, after the
freeze-drying, heating resultant glass powder for from 1 hour to 10 days
in an oxidizing atmosphere at a temperature of from 250.degree. C. to the
glass transition temperature T.sub.g.
14. A process according to claim 13, wherein the glass powder is heated for
from 12 to 48 hours at a temperature below the glass transition
temperature T.sub.g of from 400 to 600.degree. C. in air.
15. A process according to claim 2, wherein the grinding elements have a
size of from 0.5 to 2 mm.
16. A process according to claim 4, wherein the grinding liquid has a pH of
from 8 to 12.
17. A process according to claim 15, wherein the grinding liquid is a
mixture of 80-99% by weight water and 1-20% by weight methanol, ethanol,
n- or i-propanol, acetone or tert.-butyl alcohol.
18. A process according to claim 16, wherein the grinding liquid is a
mixture of 80-99% by weight water and 1-20% by weight methanol, ethanol,
n- or i-propanol, acetone or tert.-butyl alcohol.
19. A process according to claim 18, further comprising heating the glass
powder for from 12 to 48 hours at a temperature of from 400 to 600.degree.
C. in air.
20. In a process for the preparation of glass powder by wet grinding a
mixture of particulate glass and a grinding liquid, the improvement which
comprises freeze-drying resultant slurry to remove the grinding liquid.
21. A process according to claim 1, further comprising heating resultant
glass powder at a sufficiently high temperature and for a sufficiently
long time in an oxidizing atmosphere to remove any gray color and lighten
the glass.
22. A process according to claim 20, further comprising heating resultant
glass powder at a sufficiently high temperature and for a sufficiently
long time in an oxidizing atmosphere to remove any gray color and lighten
the glass.
23. A process for the preparation of very fine glass powder of high purity
having a desired mean particle size d.sub.50 of .ltoreq.10 .mu.m by
wet-grinding in the presence of grinding elements, comprising grinding a
glass powder having a higher mean particle size than 10 .mu.m and a
maximum particle size of .ltoreq.300 .mu.m to said desired particle size
in a stirred mill with glass grinding elements, in the presence of a
grinding liquid comprising a mixture of at least 50% by weight of water
and at least one water-soluble, oxygen-containing organic compound having
1 to 5 carbon atoms in the molecule; freezing the ground slurry; and
freeze-drying the frozen ground slurry to remove the grinding liquid.
24. A process for the preparation of very fine glass powder of high purity
having a desired means particle size (d.sub.50) of .ltoreq.10 .mu.m by wet
grinding in the presence of grinding elements, comprising:
(a) grinding a glass powder having a means particle size greater than 10
.mu.m and a maximum particle size .ltoreq.300 .mu.m to said desired means
particle size (d.sub.50) of .ltoreq.10 .mu.m in a stirred mill with glass
grinding elements, in the presence of a grinding liquid comprising water
or a mixture of at least 50% by weight of water and at least one water
soluble oxygen-containing organic compound having 1 to 5 carbon atoms in
the molecule;
(b) freezing the ground slurry;
(c) freeze-drying the frozen ground slurry to remove the grinding liquid;
and
(d) heating the resultant glass powder at a sufficiently high temperature
and for a sufficiently long time in an oxidizing atmosphere to remove any
gray color and lighten the glass.
25. In a process for the preparation of a glass powder by wet grinding a
mixture of particulate glass and a grinding liquid, the improvement which
comprise:
(a) freeze-drying the resultant slurry to remove the grinding liquid; and
(b) heating the resultant glass powder at a sufficiently high temperature
and for a sufficiently long time in an oxidizing atmosphere to remove any
gray color and lighten the glass.
Description
BACKGROUND OF THE INVENTION
This invention relates to the preparation of very fine glass powder of high
purity having a mean particle size d.sub.50 of .ltoreq.10 .mu.m by
wet-grinding in the presence of grinding elements.
High-purity glass powders are required, in particular, as fillers for
plastics employed in the dental sector, for example, dental fillings. For
glass powders of this type, mean particle diameters d.sub.50 of at most 10
.mu.m, preferably <5 .mu.m, in particular .ltoreq.3 .mu.m are required
since the mechanical properties such as polishability and abrasion
resistance, are improved with increasing fineness (decreasing particle
diameter). Excessively large glass particles (<10 .mu.m) produce a rough
surface in the cured plastic or break out and leave holes and sharp edges.
The refractive index of the glass powder must agree very closely with that
of the plastic in order to achieve high transparency and translucency of
the filled plastic. If the glass powder contains, for example, coloring
particles or particles having different refractive indices, the
translucency and transparency and possibly also the color of the filled
plastic are impaired, so that the plastic can frequently only be used with
considerable restrictions, if at all.
Glass powders are prepared by grinding. The grinding processes hitherto
have the disadvantages of, in some cases, high energy consumption for the
grinding, long grinding times for fine particle sizes and high abrasion of
grinding pebbles and the mill wall. The abrasion particles impair the
transparency and translucency of the filled plastic and make the
production of very pale tooth colors difficult.
The conventional dry-grinding processes are at the limit of their
performance for these small particle sizes, require long grinding times
and generally require an additional air separator for classifying the
grinding material. Abrasion of the grinding elements, wear of the grinding
container or of the air separator and the energy consumption are so high
that these grinding processes are unsuitable for the preparation of very
fine glass powders.
Although wet-grinding processes using water yield fine particle sizes in a
shorter time than do dry-grinding processes, the grinding elements are
still subject to considerable abrasion and a particular disadvantage is
that numerous agglomerates, i.e., very solid clusters of powder particles,
which act in a similar way to large individual particles and dramatically
impair the properties of the filled plastic, form from the grinding slurry
on drying. If, by contrast, the grinding is carried out in the presence of
organic liquids in which agglomeration is substantially suppressed on
drying (for example, low-boiling hydrocarbons), the grinding times are
considerably extended, the amount of grinding abrasion increases
correspondingly and additional safety precautions for example, explosion
protection, become necessary.
SUMMARY OF THE INVENTION
An object of the invention is to provide an improved process for the
preparation of very fine glass powder of high purity, especially a process
wherein powders having a mean particle size d.sub.50 of from 0.2 to 10
.mu.m, preferably from 0.5 to 5 .mu.m, in particular from 0.5 to 2 .mu.m,
can be produced.
Another object is to provide resultant glass powders having a purity which
permits their introduction into filled plastics for providing very pale
tooth colors in the dental sector.
Upon further study of the specification and appended claims, further
objects and advantages of this invention will become apparent to those
skilled in the art.
To achieve these objects, there is provided a process having several novel
concepts, each novel concept contributing to the success of the process.
In combination there is provided a comprehensive process for the
preparation of very fine glass powder of high purity having a desired mean
particle size d.sub.50 of .ltoreq.10 .mu.m by wet-grinding in the presence
of grinding elements, comprising grinding a glass powder having a higher
mean particle size than 10 .mu.m and a maximum particle size of
.ltoreq.300 .mu.m to said desired particle size in a stirred mill with
glass grinding elements, in the presence of a grinding liquid comprising
water or a mixture of at least 50% by weight of water and at least one
water-soluble, oxygen-containing organic compound having 1 to 5 carbon
atoms in the molecule; freezing the ground slurry; and freeze-drying the
frozen ground slurry to remove the grinding liquid.
The process is preferably carried out using an attrition or stirred mill
(Attritor mill; see Perry's Handbook of Chemical Engineering, Sixth
Edition, McGraw-Hill, New York, 1984, pp. 8-35), since a mill of this type
allows glass powders having the desired fineness to be prepared
particularly simply. In order to achieve short grinding times, it is
furthermore generally necessary to carry out the grinding in the presence
of a grinding liquid comprising water or preferably mixtures of at least
50% by weight of water and at least one water-soluble, oxygen-containing
organic compound having 1 to 5 carbon atoms in the molecule. Mixtures of
water with organic compounds are preferred since attack of water on the
glass powder is less pronounced. More preferably, the content of the
organic compounds in the aqueous mixtures is 1-20%, especially 1-5%, of
the organic compound. Suitable organic compounds include, but are not
limited to, aldehydes, e.g., formaldehyde, acetaldehyde, propionaldehyde,
butyraldehyde and pentanal; ketones, e.g., acetone, methyl ethyl ketone
and diethyl ketone; ester, for example, ethyl acetate, methyl acetate,
propyl acetate, methyl formate, ethyl formate and propyl formate; or
acids, e.g., acetic acid and propionic acid. Monohydric, dihydric and
trihydric alcohols are also suitable. An example of a suitable trihydric
alcohol is glycerol which, however, requires longer evaporation times;
examples of suitable dihydric alcohols are ethylene glycol and the propane
diols. Monohydric alcohols, in particular those having up to five carbon
atoms in the molecule, are particularly suitable. Of the eight isomeric
pentanols, most can only be used mixed with lower alcohols since their
water solubility, up to that of 2-pentanol, is inadequate. Their use is
also associated with disadvantages due to, in some cases, an unpleasant
odor. Of the 4 isomers of butanol, some are likewise only moderately
soluble in water, so that they can again only be used mixed with other
alcohols. However, tert.-butyl alcohol is highly suitable, both due to its
good water solubility and also due to its high melting point.
Of the organic compounds, those are preferred having a boiling point not
above 100.degree. C.; since, otherwise, the removal thereof from the
frozen grinding slurry during drying takes too long. Particular preference
is given to acetone, tert.-butyl alcohol, methanol, ethanol and n-and
i-propanol. Particularly good grinding results are achieved using mixtures
of organic compounds, especially the particularly preferred compounds with
from 80 to 99% by weight of water. The proportion of water in the mixture
should preferably be selected so that the mixture has a freezing point of
above -40.degree. C. since the operating costs for freezing equipment
which operates at lower temperatures is disproportionately high.
It has furthermore proved advantageous for the grinding operation to be
carried out within a pH range of from 1 to 12. Acid or alkaline attack on
the glass can take place outside these limits. It is particularly
advantageous to work either in the acidic range, i.e., at a pH of from 1
to 6, in particular from 3 to 6, or in the alkaline range, i.e., at a pH
of from 8 to 12, in particular 8 to 11. At these pH values, the viscosity
of the grinding slurry drops to lower values. A lower viscosity of the
slurry is advantageous since the proportional amount of energy directly
attributable to the comminution of the glass increases with respect to the
overall grinding energy expended and, conversely, the amount used for
"stirring" the slurry decreases, so that the energy efficiency of the
grinding performance increases. The pH can be adjusted using any desired
acids and bases so long as they react with the glass only slightly, if at
all. However, preference is given to acid and bases which can also easily
be removed from the grinding slurry, i.e., readily volatile acids and
bases, e.g., acetic acid, HCl, HNO.sub.3, NH.sub.3, methylamine,
dimethylamine, ethylamine, diethylamine, etc. Preference is given to HCl,
HNO.sub.3, NH.sub.3 and ethylamine.
In order to avoid impairing the properties of the resultant glass powder
with respect to color, transparency and translucency in the processed
state, there are employed grinding elements made of a glass having
abrasion characteristics such that only an insignificant impairment
occurs, if any at all, in the properties of the resultant glass powder.
Optical and mechanical properties, such as refractive index, color,
hardness, resistance to hydrolysis, polishability, etc., of the glass
employed for the grinding elements should be similar or preferably
identical to the corresponding properties of the glass to be ground. It is
preferred for the grinding elements and the glass to be ground to have the
same composition.
For grinding in the stirred mill (Attritor mill), the material to be ground
must be precomminuted to a maximum particle size of .ltoreq.300 .mu.m,
preferably .ltoreq.200 .mu.m. This precomminution can expediently be
effected by dry-grinding of the glass in a ball mill, in which particles
of the maximum sizes can be produced rapidly and without measurable
abrasion of the grinding container and the grinding balls.
The fine grinding of the precomminuted glass powder to the desired size of
the stirred mill is effected using grinding elements having a size of from
0.3 to 10 nun. If the grinding elements are larger than 10 mm, the
resultant grinding times are very long and the wear of the grinding
elements and of the mill furthermore increases considerably- During
grinding in the stirred mill, the glass to be ground is pumped through the
mill as a slurry (suspension), and the grinding elements are retained by
various conventional measures, for example, a filter cartridge or a
friction gap of appropriate dimensions. If the grinding elements are
smaller than 0.3 mm, there is a danger of them no longer being adequately
retained and damaging the retention systems.
The number of grinding elements affects the grinding action and thus the
grinding time necessary in order to prepare a powder having a certain
particle size. At a constant weight ratio of grinding element to grinding
material, the number of grinding elements, and thus the number of contact
points between which the glass particles are ground, increases with
decreasing size, and the grinding time decreases. Grinding elements having
a size of from 0.5 to 2 mm are therefore preferred. The grinding elements
may be in the form of balls, cylindrical elements or glass fragments. The
cylindrical shape of the grinding elements is preferred since this shape
allows an optimum grinding result to be achieved. These grinding elements
can be obtained from sections of a suitable glass rod or by sintering
preforms which are dry-pressed or extruded from glass powders. The size is
taken to mean the diameter in the case of spherical grinding elements. As
for the particle size in the case of glass fragments, the length, width
and height of cubic and similar elements and the diameter and length of
cylindrical elements should be within said particle size ranges. In any
case, it is preferred for the elements to be as compact as possible, i.e.,
for the individual dimensions to be substantially identical.
In stirred mills, the grinding container, the stirrer and other
abrasion-endangered parts are generally lined with or comprise metal, in
particular hard metal, or abrasion proof ceramic, for example, Al.sub.2
O.sub.3, or porcelain. However, the abrasion of the glass powder resulting
from ceramic impairs the translucency and transparency of the filled resin
compositions based on these powders, and abrasion resulting from metal can
even result in a grey coloration. It is therefore preferred to produce
these mill parts from the glass to be ground or from a glass which has
similar properties, or in the alternative, to coat them with a glass of
this type or with an abrasion-resistant, solvent-resistant plastic. The
mechanical durability of the plastic coating can be improved by
reinforcement with glass powders or glass fibers preferably made of the
glass to be ground or a similar glass. Suitable plastics from the group
comprise the polyurethanes, aramids or chlorofluorocarbon resins are known
per se for lining mills.
When the glass powder has been ground to the desired fineness, the glass
slurry is frozen and freeze-dried. During the freeze-drying, the frozen
solvent is evaporated in a high vacuum by sublimation. The freeze-drying
is well known per se, and freeze-drying units are commercially available
from numerous manufacturers; however, the step of freeze-drying in the
production of glass powders is believed to be an entirely new concept
which is a factor leading to the success of the present invention. Since
the cost of freeze-drying units increase considerably with operating
temperatures of below -40.degree. C., it is preferred to use solvents or
solvent mixtures which freeze at temperatures not lower than -40.degree.
C. After the freeze-drying, the glass powder is in finely divided form
without agglomeration and is generally ready to use per se.
However, residues from plastic abrasion of the mill lining or from the
solvents used may be present in the glass powder, in some cases, strongly
adsorbed by the glass surface and, in some cases, resulting in the glass
powder which is unusable for very pale tooth colors. In such cases, and
also in general, if particularly pure glass powders are to be produced,
the glass powder is heated, after the freeze-drying at a sufficiently high
temperature and for a sufficiently long time in an oxidizing atmosphere to
remove any gray color and brighten the glass, e.g., for from 1 hour to 10
days normally in air, at temperatures between 250.degree. C. and the glass
transition temperature T.sub.g of the glass powder, during which treatment
the organic constituents are oxidized. The precise heating time depends on
the temperature to which the glass powder is heated and on the tenacity
with which the organic constituents are adsorbed by the glass powder, and
should expediently be matched to the particular grinding conditions. Good
results are generally obtained using treatment times of from 12 to 48
hours at from 400 to 600.degree. C.
The process permits the production of very pure glass powders having mean
particle sizes d.sub.50 of from 0.2 to 10 .mu.m, without difficulty. The
particle sizes are determined, for example, using laser diffraction or
sedimentation methods (DIN 66 111). Glass powders of this type are
suitable for a variety of uses, for example, for the production of
sintered glass ceramics. They are particularly useful in dental technology
for the production of filled synthetic resins; for this application, the
surface of the glass powder particles is very often treated in a manner
known per se with suitable silanes, for example, chlorosilanes, in order
to obtain better mechanical and chemical binding of the glass powder in
the resin composition. Synthetic resins in dental technology are
preferably filled with glass powders having mean particle sizes of from
0.5 to 3 .mu.m, in particular from 0.5 to 1.5 .mu.m. Although the process
also allows mean particle sizes of less than 0.2 .mu.m to be produced, the
advantages of such a lower particle size range are relatively minor,
whereas the grinding operation becomes very lengthy and, in general, is no
longer economically worthwhile.
This invention is applicable to the production of all types of glass
powder, irrespective of the composition thereof. Such compositions are
known in the literature, for example, the powders used in dental
compositions. See, also, for example, the numerous types of glass that are
manufactured in powder form in Schott's Product Information Brochure No.
40001d for technical glass. Accordingly, this invention does not depend on
any particular glass composition.
Without further elaboration, it is believed that one skilled in the art
can, using the preceding description, utilize the present invention to its
fullest extent. The following preferred specific embodiments are,
therefore, to be construed as merely illustrative and not limitative of
the remainder of the disclosure in any way whatsoever.
In the foregoing and in the following example, all temperatures are set
forth uncorrected in degrees celsius; and, unless otherwise indicated, all
parts and percentages are by weight.
The entire disclosure of all applications, patents and publications, cited
above, and of corresponding German Application P 41 00 604.6, are hereby
incorporated by reference.
EXAMPLE
20 kg of a glass powder of a particle size less than 200 .mu.m and an
average particle size of about 40 .mu.m and 30 1 suspension liquid,
consisting of a 95 wt% water and 5 wt% 1-propanol were introduced into a
stirred vessel and pumped continuously through a stirred ball mill having
a capacity of 12.5 1, which was previously filled with 18 kg glass
particles of a cylindrical shape with a diameter of 1.3 mm and a length of
1.3 mm as a grinding medium. The grinding medium is prevented from leaving
the stirred ball mill by a gap of 0.3 mm, which is part of the stirred
ball mill. The impeller stirs the media at a velocity of 1400 rpm. The
impeller and the interior of the vessel and the mill have a resilient
coating of a polyurethane resin. After an operating time of 8 hours, the
slurry is freeze-dried at -40.degree. C. The resulting powder has an
average particle size of about 0.7 .mu.m, with all particles less than 3
.mu.m.
For a special purpose, this glass is heated in an oxidizing atmosphere
(air) for 48 hours at a temperature of 500.degree. C. to yield an
extremely pure and bright powder. The mill that was used is a Drais
Ruhrwerksmuhle Type PM 12.5 RLV.
The chemical composition of the glass powder is (in weight-% on oxide
basis): SiO.sub.2 55; BaO 25; B.sub.2 O.sub.3 10; Al.sub.2 O.sub.3 10.
The preceding example can be repeated with similar success by substituting
the generically or specifically described reactants and/or operating
conditions of this invention for those used in the preceding examples.
From the foregoing description, one skilled in the art can easily ascertain
the essential characteristics of this invention and, without departing
from the spirit and scope thereof, can make various changes and
modifications of the invention to adapt it to various usages and
conditions.
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